1. Field of the Invention
This invention relates to a process for forming a silver coating on a surface of a vitreous substrate, in particular to the silvering of glass, that is to say the chemical deposition of a coating of silver, using a silvering solution.
Such a metal coating may be deposited pattern-wise to form a decorative article, but the invention has particular reference to glass substrates bearing a continuous reflective coating. The coating may be applied to a substrate of any form, for example to an artistic object, to achieve some desired decorative effect, but it is envisaged that the invention will find greatest use when the coating is applied to a flat glass substrate. The reflective coating may be so thin that it is transparent. Glass panes bearing transparent reflective coatings are useful inter alia as solar screening panels or as low-emissivity (in respect of infra-red radiation) panels. Alternatively, the coating may be fully reflective, thus forming a mirror-coating. Such a process is also used for the formation of silvered glass microbeads (that is to say microbeads carrying a coating of silver), which may for example be incorporated in a plastics material matrix to form a reflective road-marking paint or a conductive plastics material.
2. Description of the Related Art
Conventionally, silver mirrors are produced as follows. The glass is first of all polished and then sensitised, typically using an aqueous solution of SnCl2. After rinsing, the surface of the glass is usually activated by means of an ammoniacal silver nitrate treatment. The silvering solution is then applied in order to form an opaque coating of silver. This silver coating is then covered with a protective layer of copper and then one or more coats of paint in order to produce the finished mirror.
The silver coating does not always adhere sufficiently to the substrate. In the case of certain prior products, it has been observed that the silver coating comes away spontaneously from the glass substrate. This is, for example, the case when silvered microbeads manufactured in a normal manner are incorporated in a plastics matrix.
The aim of the invention is to improve the adhesion of such a silver coating to the glass and thus to improve the durability of this silver coating.
According to a first aspect of the invention, there is provided a process for forming a silver coating on a surface of a vitreous substrate, comprising an activating step in which said surface is contacted with an activating solution, a sensitising step in which said surface is contacted with a sensitising solution, and a subsequent silvering step in which said surface is contacted with a silvering solution comprising a source of silver to form the silver coating, characterised in that said activating solution comprises ions of at least one of bismuth (III), chromium (II), gold (III), indium (III), nickel (II), palladium (II), platinum (II), rhodium (III), ruthenium (III), titanium (III), vanadium (III) and zinc
The characteristic of the invention therefore is to xe2x80x9cactivatexe2x80x9d the substrate by treating it with a specific activating solution before silvering.
It has been observed that the treatment of glass using an activating solution according to the present invention improves the adhesion of the silver coating.
The sensitising step contributes to improving the adherence of the silver coating and therefore its durability. Preferably the sensitising step is carried out before said silvering step. This sensitising step is typically carried out with a sensitising solution comprising tin (II) chloride.
Preferably, said sensitising step is carried out prior to the activating step. We have observed that the order of the steps is important to obtain good durability. This observation is very surprising because the activation treatment does not really produce a distinct continuous layer containing bismuth (III), chromium (II), gold (III), indium (III), nickel (II), palladium (II), platinum (II), rhodium (III), ruthenium (III), titanium (III), vanadium (III) or zinc (II), but they are in the form of islets on the surface of the glass. An analysis of the surface of glass treated with a sensitising solution containing tin (II) chloride followed by an activating solution containing palladium (II) shows the presence of a certain proportion of palladium atoms with respect to tin atoms at the glass surface. Typically, one finds about 0.4 atoms of palladium per atom of tin, and 0.3 atoms of tin per atom of Si at the surface of the glass.
The activation treatment according to the invention may be effected on various types of vitreous substrates, for example on glass microbeads. It has been observed that the treatment according to the invention improves the adhesion of the silver coating subsequently deposited on the glass microbeads. When such silvered microbeads are incorporated in a plastic, it is found that the coating of silver has less of a tendency to peel away from the bead than if the activation treatment according to the invention is omitted. The invention can also be implemented on flat glass substrates, and it is believed that the invention will be particularly useful for this type of substrate. Consequently, the treatment is preferably effected on a flat glass substrate, such as a glass sheet.
The layer of silver may be deposited in the form of a silver coating which is fairly thin so that it is transparent. Flat glass substrates carrying such transparent coatings are used to form glazing panels which reduce the emission of infrared radiation and/or which protect from solar radiation. Thus according to one embodiment of the invention the thickness of the layer of silver formed in said silvering step is between 8 nm and 30 nm.
However, the treatment is preferably applied to glass substrates onto which a thick opaque silver coating is subsequently applied in order to form a mirror. Such embodiments of the invention, where the product is a mirror, are used for example as domestic mirrors or as vehicle rear-view mirrors. The invention makes it possible to produce mirrors on which the silver coating has an improved adhesion to the glass. Thus according to another embodiment the thickness of the layer of silver formed in said silvering step is between 70 nm and 100 nm.
According to the present invention, the activation of the glass is effected before silvering by treating the glass substrate with a specified activating solution. It is observed that the silver coating of the mirror produced in this way has better adhesion than that of a mirror manufactured by the conventional process.
The improvement of the adhesion of the silver coating obtained by the process according to the present invention is observed in different ways.
The adhesion of a silver coating to its glass substrate may be assessed quickly by testing using adhesive tape: an adhesive tape is applied to the silver coating and then pulled off. If the silver coating is not adhering well to the glass, it comes away from the glass when the tape is pulled off.
The degree of adhesion of the silver coating to the glass can also be observed by subjecting the product to an accelerated ageing test such as the CASS Test or Salt Fog Test. It is sometimes found that the product subjected to such tests has a certain edge corrosion and/or light diffusing specks (xe2x80x9cwhite specksxe2x80x9d).
The activation treatment according to the invention affords another advantage. We have observed that the silvering reaction on glass activated according to the invention is more effective, that is to say the reaction yield is greater. It is possible to achieve yields improved by around 15% compared with silvering effected on a glass activated in a conventional manner, with a solution of ammoniacal silver nitrate. This presents advantages from the economic point of view since one can use less reagents to form the same thickness of silver coating and also from the environmental point of view since the quantity of waste from the silvering reaction to be eliminated can be reduced.
It is conventional to protect the silver coating with an overcoating of copper to retard tarnishing of the silver layer. The copper layer is itself protected from abrasion and corrosion by a layer of paint. Those paint formulations which afford the best protection against corrosion of the copper layer contain lead pigments. Unfortunately lead pigments are toxic and their use is being increasingly discourage for reasons of environmental health.
It has recently been proposed to protect the silver coating by treatment with an acidified aqueous solution of Sn (II) salt (see British patent application GB 2252568). According to another recent proposal, the silver coating is protected by treatment with a solution containing at least one of Cr (II), V (II or III), Ti (II or III), Fe (II), In (I or II), Cu (I) and Al (III) (see British patent application GB 2254339). We have observed that the activation treatment according to the present invention is particularly useful for the manufacture of such products. One important application of the protection treatments according to GB 2252568 and GB 2254339 is the formation of silver mirrors which do not include a conventional protective layer of copper. Such mirrors can be protected with lead-free paints. The activation treatment according to the present invention is particularly advantageous for the manufacture of such mirrors. This is because the activation treatment of the glass during the manufacture of mirrors protected with such treatment significantly improves the adhesion of the silver coating of such mirrors and therefore their durability. Consequently, the invention applies preferably to the manufacture of mirrors with no copper layer, and in particular to mirrors formed by a process in which the silver coating is subsequently contacted with a solution containing ions of at least one of the group consisting of Cr (II), V (II or III), Ti (II or III), Fe (II), In (I or II), Sn (II), Cu (I) and Al (III).
The glass substrate may be brought into contact with the activating solution by dipping in a tank containing an activating solution but, preferably, the glass substrate is brought into contact with the activating solution by spraying with a solution containing an activating solution. This is particularly efficacious and practical in the case of flat glass substrates, for example during the industrial manufacture of flat mirrors, in which sheets of glass pass through successive stations where sensitisation, activation and then silvering reagents are sprayed.
We have observed that the glass substrate may be effectively activated by a rapid treatment using the specified activating solution. It has been observed that the glass/activating solution contact time may be very short, for example around a few seconds only. In practice, in the industrial production of flat mirrors, the sheet of glass moves along a mirror production line on which the glass passes through an activation station where the activating solution is sprayed, then through a rinsing station and afterwards through the silvering station.
The activating solution preferably comprises a source of palladium, most preferably a palladium (II) salt in aqueous solution, in particular PdCl2 in acidified aqueous solution.
The activating solution may be used very simply and economically. The PdCl2 solution may have a concentration of from 5 to 130 mg/l. We have observed that bringing the glass substrate into contact with a quantity of from 1 to 23 mg, preferably at least 5 mg of PdCl2, per square meter of glass is entirely sufficient to activate the glass substrate effectively. In fact, we have observed that the use of quantities of PdCl2 higher than about 5 or 6 mg PdCl2/m2 does not afford any significant improvement. Therefore it is preferred to treat the glass substrate with about 5 or 6 mg of PdCl2 per square meter of glass.
We have found that best results can be obtained when the pH of said activating solution is from 2.0 to 7.0, most preferably from 3.0 to 5.0. This pH range allows solutions to be formed which are both stable and effective for activating the glass. For example, when using palladium, below pH=3.0 the level of palladium deposited on the glass substrate may be reduced, leading to a poor quality product. Above pH=5.0, there is a risk of precipitation of palladium hydroxide.
According to a second aspect of the invention, there is provided a mirror comprising a vitreous substrate carrying a silver coating which is not covered with a protective layer of copper, the mirror exhibiting an average number of white specks of less than 10 per dm2, preferably less than 5 per dm2, after having been subjected to the accelerated ageing CASS Test and/or the Salt Fog Test defined below. Such a silver mirror without a copper layer is advantageous since the silver coating adheres well and has good durability.
The silver coating may be covered with one or more protective paint layers and according to a preferred aspect of this invention such a paint is free, or substantially free, of lead. Where more than one such paint layer is used, the paint layers other than the uppermost paint layer may contain lead. However, for environmental health reasons, lead sulphate and lead carbonate in the lower paint layers are preferably absent so that where lead is present in these lower layers it is preferably in the form of lead oxide.